The present invention is related to a power train and more particularly to an auxiliary creeper drive system therefor.
Most specialized vehicles utilize multiple ratio and reversible transmissions to afford the operator a selection of ranges in which he can best operate it. However, the cost of these transmissions generally increases directly with the number of ratios employed, and consequently it is desirable to minimize the number of ratios for reasons of economy. In the case of a material handling fork lift truck, for example, a hydraulic torque converter is frequently coupled between the engine and one of these more economical reversible transmissions in order to broaden the relatively low number of operating ranges of the power train. In addition, even though the speed of the engine is varied by the operator when the transmission is in a specific ratio, it is necessary in many cases to provide sophisticated modulating control systems for creeping operation of the lift truck while operating the engine at full throttle conditions which is desirable for operation of auxiliary equipment. Specifically, precise inching of specialized work vehicles has heretofore been accomplished at full engine speed by a manually operated control system which modulatably decreases the fluid pressure to one of the engaged disc-type clutch actuating pistons to slip the interleaved clutch plates and discs of that clutch in the power train. Thus, despite the operating speed of the engine, the degree of slip of a transmission clutch can be controlled to decrease the output speed of the power train and the ground speed of the vehicle for inching purposes.
While many of the aforementioned clutch slipping systems are successfully being used by the industry, they represent costly additions to the power train. Not only are the control systems therefor complex, but also the slipping clutches must be enlarged and be extensively engineered in order for them to withstand the partially engaged, heat-generating conditions for relatively extended working periods. Moreover, even with complicated modulating systems for the clutches or torque converter transmissions, the ability to obtain precise and positive control for inching or creeping is not as good as desired.